Our long-term goals are: a) to characterize the molecular mechanisms of cardiac gap junction regulation and b) to identify the consequences of this regulation on the function of the heart. Our previous studies have centered on the structural bases for the regulation of Connexin43 (Cx43), the most abundant cardiac gap junction protein. Using exogenous expression systems, we have shown that the carboxyl terminal region of Cx43 (Cx43CT) acts as a regulatory domain. Here, we propose that: a) the integrity of the Cx43CT domain is essential for the regulation of native cardiac gap junctions and b) this regulation plays a key role in specific morphological and electrophysiological changes that follow an ischemic event. Studies on cardiac gap junction regulation have been hindered by the lack of an appropriate experimental model where the function of the CT domain can be directly and specifically altered. Recently, the Willecke laboratory developed a "knock-out/knock-in" mouse line where the gene coding for wild-type Cx43 was replaced with a truncated form that lacks most of the CT domain. These mice present us with the first biological system to document directly the role of Cx43CT in the physiology of the heart. Specifically, we will characterize the role of the Cx43CT domain on: 1) the biophysical properties and pH gating of cardiac gap junctions. 2. The internalization of Cx43 in response to low intracellular pH (pHi) or global ischemia. 3. The electrophysiological behavior of adult murine ventricle in response to low pHj and ischemia. Finally, we will determine the ability of the CT fragment to act as an independent domain within the environment of a living organism. Overall these studies will offer data fundamental to our understanding of the molecular mechanisms of regulation of cardiac gap junctions and their role in heart function in health and disease.